This invention relates to radiation energy detectors and particularly to radiation transducer systems for surveying and precisely locating radiation sources within the human body.
Nuclear medicine is one of the most rapidly developing fields of clinical investigation. The term deriving from its origin in nuclear physics involves administration by injection into a vein of a small dose of radioisotope (a radioactive substance emitting gamma rays). The bloodstream distributes the dose throughout the body and a suitably sensitive transducer records a history of this distribution.
Areas of the body having high "uptake" of the isotope or a rich blood supply show up as bright or highly illuminating sources while conversely those of low "uptake" or blood supply appear dark. In this manner any portion of the body or a specific organ may be subjected to clinical investigation in a safe, reliable and non-invasive manner.
The device most frequently employed in nuclear investigation is a radiation transducer having a scintillation crystal (i.e. one that emits light photons proportionately to received radiation energy quanta). A plurality of phtototubes in close optical communication with the crystal generate electrical signals responsive to the emitted light. U.S. Pat. No. 3,011,057, incorporated herein by reference, discloses a radiation imaging device generally referred to as an "Anger" (inventor's name) or gamma ray camera.
A scintillation camera of this type produces a picture of the isotope distribution by detecting individual gamma rays passing through a suitable collimator and striking the crystal. Electronic circuitry interprets and translates the phototube outputs into orthogonal (X,Y) coordinates and a third signal (Z) representative of the resultant camera signal output proportional to the energy level for each gamma ray event. If the energy Z signal is of acceptable magnitude i.e. falling with selected high and low values (Z window) the event is recorded and an image point is depicted in a two dimensional matrix in accordance with its coordinate position. Ordinarily the positional coordinate and energy level signals are analog but through well known techniques may be converted to their digital equivalents.
With advances in nuclear medicine and increase use as a diagnostic tool, attempts are being made to acquire increased and improved information from gamma cameras e.g. in the recognition of small tumors, measurement of heart function and dimensions etc. Unfortunately with this effort the inherent non-linearities of camera design and construction i.e. spatial distortion of image points, become more recognizable and deleterious. It is further true that with newer camera designs intended to improve camera spatial resolution both non-linearity and non-uniformity of image (the non-uniform response of camera output signal to a flood field source providing substantially uniform radiation across the camera field) have increased rather than diminished.
The obviate these inherent sources of error the prior art discloses various corrective measures. U.S. Pat. No. 3,745,345, incorporated herein by reference, determines the magnitude of camera non-linearity for a number of specific accurately located phantom radiation image points. From this, X and Y increments are derived and stored for employment in correcting camera signals either on-line or after the original distorted image has been located in core for subsequent display. While this technique potentially introduces some degree of improvement it has not sufficiently accomplished its purpose as to warrant its use and unfortunately apparently adds artifacts to the final image. Moreover the calibration technique of this patent is difficult and laborious to accomplish with any degree of precision and accuracy. It is further important to note that this prior art system completely ignores differences in Z (energy level) signal, as a function of the source position i.e. the Z signal output responsive to a point source of radiation at a particular position on the camera face, as will be made clear this is of significance.
U.S. Pat. Nos. 3,937,964; 3,980,886 and 4,001,591 all present other approaches toward increasing resolution (ability to recognize radiation sources) and avoidance of non-linearity, but none presents a system with the economy, reliability and capability for such purpose as that contained in the present novel concept. Other published scientific papers disclosing information pertinent to this subject and concerned with possible corrective measures are "Online Digital Methods for Correction of Spatial Energy Dependent Distortions of Anger Camera Image", Dennis Kirch, Leonard Shabason, Michael LaFree, and Gerry Hine and "Quantitation Studies with the Gamma Camera after Correction for Spatial and Energy Distortion" by F. Saussaline, A. Todd-Poknopek and C. Raynaud.
It is therefore an object of this invention to provide a radiation transducer imaging system having high image resolution with minimal spatial non-linearity and signal non-uniformity. Another object of the invention is to provide a system wherein each detected energy event is corrected to its true spatial location. Another object of the invention is to provide a system in which event detection is controlled as a function of spatial location. Another object is to produce a gamma ray detection system having uniform point source response and enhanced resolution. Yet other objects of the invention are to provide means for calibrating a radiation imaging system and methods for accomplishing the foregoing tasks.